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After the formation of the Isthmus of Panama, about 2-5 Ma, a massive interchange between the previously separated mammalian faunas of South and North America took place. Afterwards, during the Late Pleistocene (Lujanian Land Mammal Age)—Holocene transition (less than 10000 years BP), many of the taxa originally present in South America became extinct. Here, we report results of a statistical assessment of the relative importance of factors potentially associated with extinctions. Several factors (namely trophic niche, origin, and body size) were tested for their association with the probability of extinction, but body mass was the only factor found to be significantly correlated with the probability of extinction (P< 0-0001). The reduction in deviance with the inclusion of body mass was 55-7 per cent. The fate of 85-6 per cent. of the 120 Late Pleistocene mammalian genera included in the analyses was in …